The invention relates to transmission systems for the polarisation-insensitive transmission of signals over a signal route.
The article entitled "Polarization-Switching Techniques for Coherent Optical Communications", published in Journal of Lightwave Technology, Volume 6, No. 10, October 1988, pages 1537 to 1548 inclusive, discloses a unidirectional transmission system with polarisation-intensitive transmission of signals over an optical fibre between a transmitter and a receiver.
It is generally known that at the end of an optical waveguide or optical fibre the polarisation state of the light transmitted over the optical waveguide fluctuates. This can be attributed to various causes, such as a mechanically twisted optical fibre, temperature variation, bends in the optical fibres and the like. Because of the varying mismatch between the polarisation of the received signal and the polarisation of the signal from the local oscillator, the sensitivity of the receiver is adversely affected. In the abovementioned article a number of solutions are mentioned for achieving a transmission over an optical fibre which as far as possible is insensitive to the varying polarisation. One of the solutions is the polarisation switching method, using a high-order retarder, in which use is made of a birefringent medium.
From the abovementioned article it is known that a high-order retarder (preferably a birefringent optical fibre) may be provided at the transmission side of a coherent communication system, in order to make this system insensitive to polarisation by applying the principle of "data-induced polarisation switching". When a receiver is shared by a number of transmitters, it would be advantageous to apply said method at the receiver side. However, this method only works if the polarisation of the signal supplied to the high-order retarder is at a fixed angle of approximately 45.degree. relative to the main axis of the high-order retarder. In practice, unfortunately, the signal supplied to the receiver has on the contrary, a strongly fluctuating polarisation, which will consequently produce problems if the high-order retarder is used on the receiver side without further measures being taken.
Dutch Patent Application 9002713 in the name of the Applicant describes a unidirectional transmission system in which the high-order retarder is connected at the receiver side between the optical fibre and the coupling device. This is possible because the angle between one of the main axes of the retarder and the polarisation of the local oscillator is greater than 0.degree. and less than, or equal to, 180.degree. according to the Poincare representation. Preferably this angle is 90.degree. according to the Poincare representation. In the case of a linearly polarised signal from the local oscillator and a high-order retarder with linear retardation, this angle of 90.degree. according to the Poincare representation, corresponds to an angle of 45.degree. between the polarisation axis of the local oscillator and the main axes of the retarder.